Wafer inspection machines typically permit a wafer to be placed on top of a rotating chuck with rotation speeds that exceed 4000 revolutions per minute being common. In some conventional systems the wafer lies flat upon a receiving surface in the wafer inspection machine which contacts substantially the entire wafer on one side. When the wafer spins the wafer remains flat. One problem with this system is that by contacting substantially the entire wafer on one side there is an increased likelihood of contaminants being introduced to the wafer on the contacting side. In addition, any contaminant caught between the two contacting surfaces will cause a deformation of the wafer surface, jeopardizing the accuracy of the defect inspection, especially for optical inspection systems.
In another conventional system, a chuck includes a vacuum corridor whose ends receive the wafer. The wafer contacts the chuck near the wafer's outer edge. The wafer is held in place at high rotational speeds (in excess of 3000 rpm) by supplying vacuum to this corridor. A problem with this system is that at high rotation speeds the air pressure above the wafer is less than the air pressure below the wafer which results in the wafer bowing upwards. Such bowing can prevent the wafer inspection machine from properly inspecting the wafer because, for example, the bowing may result in the wafer being out of focus in some areas which may prevent defects from being detected in these areas.
One conventional solution involves using a sensor on the chuck to actively measure the distance the chuck to the center of the wafer as the chuck rotates. Using this information a pressure regulator modifies the pressure beneath the wafer to reduce the bowing of the wafer. The problem with this solution is that it adds cost and complexity to the system.
What is needed is a system and method for modifying the vacuum chuck to automatically reduce the bow of an object placed thereupon without requiring any feedback mechanism.